This invention relates, in part, to a motor for rotating a magnetic disk. In particular, it relates to a motor which is used for an application in which magnetic disks, each formed with a magnetic layer on a metal disk are driven by the motor, and data is recorded on and reproduced from the magnetic disk by magnetic heads disposed near the upper and lower surfaces of each magnetic disk. Such motors are commonly referred to as disk drive motors.
The increasing use of rotating magnetic disks as a computer memory medium has created a demand for small, high speed drives which accurately rotate and position magnetic disks memories. External rotor type motors having a permanent magnetic rotor which surrounds a stator, such as a multiple pole stator, have been used in such drives. One such construction is shown in U.S. Pat. No. 4,599,664 to Schok. The stator typically has a one-phase winding which produces an alternating field and an auxiliary reluctance moment which, in cooperation with the electromagnetic torque, produces a total torque of high constancy. The rotor shaft is mounted within the stator by a ball bearing structure, with the rotor secured to the outer end of the motor shaft and defining a cantilevered bearing support The disk drive is secured to the opposite end of the motor shaft.
Disk drives require structures for supporting the cantilevered mounted rotor which provide accurate and long-life support. Various sleeve and rolling element (e.g., ball) bearing structures have been suggested for use in permanent magnet motors for disk drives.
In one ball-bearing unit, described in the background of U.S. Pat. No. 4,762,250 to Seitz, a pair of axially spaced high precision rotary ball-bearings are secured within a machined hub of the stator structure. The outer race of each bearing is fitted within the hub. The outer race of a first bearing is bonded within the hub. The second bearing is held in the hub during assembly by a special O-ring member located within an annular recess in the hub. The inner races of the bearings are fitted onto the motor shaft and generally into abutting relation with a locating shoulder on the shaft and an outer hub member on the outer end of the shaft. A steel snap-ring or an integral spacer is interposed within a machined recess in the hub between the two bearings. Suitable flat spring washers are interposed between the snap-ring and the respective opposed outer bearings of the two space-bearings to resiliently load the bearings and hold them in position with the desired accuracy and support.
The Seitz patent also discloses a cup-shaped rotor which overlies the stator and has a rotor shaft journalled in the hub by spaced sealed precision bearings.
Although rolling element bearings such as ball bearings have been and are presently widely used in commercially available motors, their use presents a number of problems. For example, ball bearings are subject to relatively fast wear and rapid damage, especially if the environment in which they are located becomes contaminated. This is, of course, true for any support structure which relies on rolling or other solid contact between the rotating and stationary members. Additionally, ball bearings are bulky and increase both the radial and verticle dimension of the disk drive significantly. For these reasons, ball bearings adversely affect both the cost and reliability of disk drives. Thus, there remains a need to improve the operating characteristics of the rotor support structure to minimize the cost.
Of course, there are many known alternatives to rolling element (ball, roller and needle) bearings. Among these are pivoting element bearings and hydrodynamic bearings. Also, the present inventor has developed a deflecting pad hydrodynamic bearing which includes a plurality of bearing pads and a support structure which supports the bearing pads for deflection in a manner which optimizes hydrodynamic wedge formation. Despite recent advances in these alternative bearing technologies, most notably the present inventor's developments in the field of hydrodynamic bearings, these technologies have not been widely used in disk drives. To some extent this may be the result of a failure to recognize the applicability of other forms of bearings to disk drives or the recent nature of improvements in bearing technology.
Another known bearing is the so-called gas-lubricated bearing. Such bearings are made by forming (typically by etching) a contoured surface onto the surface of one or both of the rotating shaft and the stationary shaft support. For manufacturing convenience, the contour is usually formed on only one of the surfaces and one of the two opposed surfaces is left smooth. A precise clearance between the support surface and the shaft, at least one of which has a contoured surface is maintained, such that, as the shaft begins to rotate, gas flowing across the contoured surface becomes turbulent and is pressurized to the extent necessary to support the shaft. If the tolerances are carefully maintained so that the bearing operates as designed, the shaft and bearing do not make contact; instead, the shaft is supported on a cushion of pressurized air. Such gas-lubricated bearing constructions can be applied to both radial bearings and thrust bearings. Further, various contoured shapes may be used including taper, taper-flat, step, pocket, spiral groove and herringbone contours. At least for thrust bearings, it is believed that a spiral groove contour is probably the best.
One of the major drawbacks of conventional gas-lubricated bearings, particularly with respect to radial bearings, is the small manufacturing tolerances required to maintain tight clearances. Since gas-lubricated bearings are typically used in small light load applications and the depth of the contour can be in the range of 1/10,000 of an inch, such spacing between the shaft and support member must be very small. In practice, it has proven difficult to economically manufacture to these close tolerances. Thus, despite the advantages they offer, gas-lubricated bearings have also not yet found wide acceptance as a substitute for rolling element and other types of bearings.
A similar drawback occurs with another known bearing commonly referred to as a plane or oilite bearing. This bearing is simply a smooth sleeve formed of a porous metal such as, for example, bronze. The porous metal is loaded with an oil or graphite lubricant. The support surface of the element moving with respect to the plane bearing simply slides on the oil or graphite lubricated surface of the plane bearing. However, in order to achieve uniform wear and to obtain accurate shaft positioning, the bearing must be manufactured to extremely close tolerances. Consequently, this type of bearing is not widely used in disk drives.
As suggested above, alternative bearing constructions have not yet been incorporated into disk drives in a commercially successful way. Thus, despite the recognized drawbacks of rolling element bearings, they are still prevalent in the disk drive industry.
Another trend within the disk drive field has been toward low-profile disk drives; this is particularly true in the lap-top computer field. Numerous attempts have been made to decrease the profile (verticle height) of disk drives without affecting performance. One example of such an attempt is that disclosed in U.S. Pat. No. 4,658,312 issued to Elsasser, et al. Constructions such as this typically employ ball-bearings and a separate shaft and rotor; these features limit the extent to which vertical height can be decreased because, for example, the top of the rotor housing must be thick enough to allow connection with the separate shaft.
Thus, there remains a need for a low-profile disk drive which is reliable, inexpensive and light weight. There is also a need for means, other than ball-bearings for supporting the shaft of a low-profile disk drive construction. Finally, there is a need for a gas-lubricated bearing which can be economically manufactured, i.e., does not have to be manufactured to close tolerances.